The present disclosure relates to a converter and an operating method thereof, and particularly, to a converter interconnected with a wind power generation farm and an operating method thereof.
In general, high voltage direct current (hereinafter, referred to as “HVDC”) refers to a high-voltage direct-current electric power transmission system, in which alternating-current power that is produced in a power plant is converted into direct-current power and then transmitted; and the converted direct-current power is converted back into the alternating-current power in a power reception region and then supplied.
HVDC is being widely and globally used in various applications from high-voltage electric power transmission to electric power distribution because of its high power transmission efficiency and low power loss.
Recently, HVDC has been recognized as an essential technique for reducing greenhouse gases and expanding the supply of new renewable energy, such as wind force and sunlight, and thus there has been an increased interest in HVDC.
In addition, HVDC has a high impact on related fields such as semiconductor power electronics, computer, control, communication, electricity, machine design, and interpretation engineering and thus has been recognized as a core technique in the power industry field on the national level.
Such a HVDC system is classified into a current-type HVDC system using a thyristor valve and a voltage-type HVDC system using an IGBT device.
The voltage-type HVDC is appropriate for a small isolated system requiring no separate power source since the voltage-type HVDC can supply reactive power as well as active power. In addition, the voltage-type HVDC is also appropriate for an offshore platform having no alternating-current power source since the voltage-type HVDC has a small area for a power conversion station and is capable of implementing a black start function.
Solutions or projects for interconnecting a large and remote new renewable energy generation farm using the voltage-type HVDC are increasingly emerging due to the above-mentioned advantages of the voltage-type HVDC.
When an alternating-current system is configured to be interconnected with a wind power generation farm, a general multi-terminal direct-current transmission apparatus is controlled by a remote control device 200.
This will be described with reference to FIG. 1.
FIG. 1 is a block diagram showing a general multi-terminal direct-current transmission apparatus.
The multi-terminal direct-current transmission apparatus of FIG. 1 is a system having four terminals, each of which is interconnected with an alternating-current system or a wind power generation farm 300 that is connected with an electric transformer 400.
There is an impedance (R+jωL) of an electric wire and an impedance of a power grid between the converter 100 and the alternating-current system.
Each terminal includes the converter 100 and is controlled by a remote control device 200.
The remote control device 200 is separated from each converter 100, and thus may control each converter 100 through communication.
When the remote control device 200 is unable to control each converter 100 due to communication failure between each converter 100 and the remote control device 200 and one or more converters 100 does not work normally, each converter 100 operates in a backup operating mode.
Thus, each converter 100 starts operating a backup controller such as a droop controller to assist an entire direct-current transmission system in continuously transmitting power.
However, when communication between the converter 100 and the wind power generation farm 300 is impossible, the converter interconnected with a wind power generator may not perform power transmission control normally.
This is because a general wind power generator is controlled through a maximum power point tracking (MPPT) scheme, excessive power is supplied to the entire direct-current transmission apparatus, and thus a common direct-current bus voltage of a multi-terminal direct-current transmission apparatus increases to make it difficult for the direct-current transmission apparatus to be continuously operated.
In addition, when a magnitude of output alternating-current power of the converter 100 is changed to correspond to a magnitude of a direct-current voltage of the entire system, inrush current may be generated in the wind power generation farm 300 interconnected with the converter 100.
Furthermore, the generated inrush current may damage devices in the power system.